1. Field of the Invention
The invention relates to a process for the preparation of polycrystalline silicon and its apparatus. Particularly, the invention relates to a process for the preparation of high-purity polycrystalline silicon deposited by thermal decomposition or hydrogen reduction of a gaseous halogenated silicon compound such as silane(SiH.sub.4), dichlorosilane(SiH.sub.2 Cl.sub.2), trichlorosilane(SiHCl.sub.3) and tribromosilane(SiHBr.sub.3) on high-purity silicon particles in a fluidized bed reactor heated by microwave and its apparatus.
2. Description of the Prior Art
Typical processes and apparatuses are disclosed in Siemens process where silicon is deposited by hydrogen reduction of trichlorosilane or dichlorosilane on silicon bar heated by electrical resistance method as described in U.S. Pat. No. 3,286,685, and Komatsu process where silicon is deposited by thermal decomposition of silane as described in U.S. Pat. Nos. 4,148,814 and 4,150,168.
In the former, silicon bar is heated to about 1000.degree. to 1200.degree. C. by resistance heating, while in the latter, it is heated to thermal decomposition temperature of about 800.degree. C. The reactors of both processes have the same type of quartz, or stainless steel bell jar which has an advantage that the reactor wall is cooled below 300.degree. C. by a coolant such as water or air so that silicon is not deposited on the inner wall, but have a disadvantage in that the polysilicon deposition rate is low, while the unit energy consumption is high because of the batch process using silicon bar which provides small surface areas for deposition.
To reduce the effects of these disadvantages, a fluidized bed process has been proposed, where silicon in the silicon-containing gas is deposited onto silicon particles while silicon particles having a large depositing area are fluidized by silicon-containing gas and carrier gas.
The fluidized bed process as mentioned above, however, generally employs an external heating method, e.g., a resistance heater as described in U.S. Pat. Nos. 3,102,861, 3,102,862, 4,207,360 and 3,963,838; Japanese Patent Laid-Open Application (KOKAI) Nos. 59-45916, 59-45917 and 57-135708, where the temperature of the reactor is higher than that of the materials to be heated, which brings about wall deposition. This heating method normally brings about a large amount of heat loss to the environment from the system, and also, it makes it very difficult to build a large diameter reactor due to limitation of the heat supply needed for CVD(chemical vapor deposition). Particularly, thermal decomposition of silane or dichlorosilane causes silicon deposition onto the inner wall of the reactor, whereby not only is reactor inner volume reduced but also heat conduction becomes worse, so that it is difficult or impossible to carry out further operations. Moreover, in the case of a quartz reactor, it may be cracked, when the reactor is cooled, due to different thermal expansion between the quartz reactor and deposited silicon (U.S. Pat. No. 3,963,838).
Internal installation of a heater instead of external heating in the system was proposed as a means of reducing the effect of the disadvantages as mentioned above. However, in the process using internal installation, silicon is deposited on the heater surface, which makes it impossible to use the process for a long time, and there still remain inherent problems related to maintenance and exchange of the heater in the case of a immersion of polysilicon resistance heater in the reactor. Particularly, internal installation of a heater is limited, since the heater itself causes some problems in making good fluidization and in eliminating contamination due to direct contact with silicon particles, and also it occupies some volume of the reactor, which reduces efficiency of the reactor and the effect of heating.